Radiological health effects of the Chernobyl accident: estimation of radiation risks

Following the Chernobyl accident of April 26, 1986, two population groups were primarily exposed to radiation: emergency workers involved in mitigation of the accident consequences (liquidators) and residents of contaminated territories. A dramatic increase in thyroid cancer incidence among residents exposed as children has been apparent since 1992. However, there remains considerable uncertainty about the existence of radiationrelated health effects in liquidators (the average dose of external radiation about 100 mGy). We studied a cohort of 71,870 Russian-resident males engaged in recovery operations within the 30-km zone between 1986-1990 for whom individual external radiation dose estimates are available (the average dose 107 mGy). A total of 58 morphologically verified leukemia cases (ICD9 204-208) were identified in this cohort between 1986 and 1998. Of these, 16 cases of chronic lymphocytic leukemia (CLL) were excluded. Even though leukemia is known to be an early consequence of acute radiation exposure, various studies indicate that CLL rates do not increase with radiation exposure. We consider the relationship between leukemia incidence rate and absorbed radiation dose in analysis based on both external and internal comparisons. Both the external (SIR=2.5, P<0.001) and internal comparisons (RR=2.2, P=0.03) indicate significantly elevated risks in the 150-300 mGy dose group. The areas worst affected by the Chernobyl accident include the Bryansk, Kaluga, Tula and Oryol oblasts of Russia. According to the Russian Commission on Radiation Safety the thyroid doses due to exposure to incorporated I in the children population of the South-West areas of the Bryansk oblast is 200-500 mGy. The paper presents results of the radiogenic thyroid cancer risk analysis in the Bryansk region among girls of 0-17 years at the time of exposure. In this group a total of 144 cases of thyroid cancer were diagnosed from 1991 to 2001, the mean thyroid dose being 80 mGy, the number of person-years in the period from 1991 to 2001 2034110. The excess relative risk per unit dose of 1 Gy (ERR1Gy) with 95% confidence interval has been estimated to be 9.1 (3.0, 23.3). The highest risk of developing thyroid cancer was established among girls of 0-4 years at the time of exposure living in the Bryansk oblast: for them the ERR1Gy is 45.3 (5.2, 9953.0). The results of epidemiological analysis obtained in the Russian National Medical and Dosimetric Registry is of major practical significance for optimizing the health care services to the population exposed to radiation as a result of the Chernobyl accident. Following the Chernobyl accident of April 26, 1986, two main population groups were exposed to radiation: emergency workers involved in mitigation of the accident consequences (liquidators) and residents of contaminated territories. A dramatic increase in thyroid cancer incidence among residents exposed as children has been apparent since 1992 [1, 2]. However, there remains considerable uncertainty about the existence of radiation-related health effects in liquidators (average dose of external radiation about 100 mGy) [3]. Our data demonstrates that there has been a significant increase in the incidence of leukemia other than chronic lymphocytic leukemia (CLL) among liquidators with external radiation dose estimates of 150 to 300 mGy. We studied a cohort of 71,870 Russian-resident males who were engaged in recovery operations within the 30-km zone between 1986-1990 and for whom individual external radiation dose estimates were available (average dose 107 mGy) [4]. A total of 58 morphologically verified leukemia cases (ICD9 204-208) were diagnosed in this cohort between 1986 and 1998, from which we excluded 16 cases of CLL. While leukemia is a well-known early consequence of acute radiation exposure [5], various studies [3] indicate that CLL rates are not increased by radiation exposure. ♣ Present address: Medical Radiological Research Center, Russian Academy of Medical Sciences, 4 Korolyov Street, Obninsk, 249036 Russia.